The invention relates in general to a diplexer. In particular, the invention relates to a diplexer fabricated using multilayer low temperature co-fired ceramic (LTCC).
With progress in communication technology, communication products are requested to be light, thin, short and small. High frequency filter circuits or switches in the front end of communication products are fabricated as ceramic devices using multilayer LCTT due to its good electrical performance on high frequency application. To further improve integration and scaling-down of ceramic devices, two main directions are focused. One is to improve the fabricating material such as increasing dielectric value and reducing dielectric thickness of capacitors. The other is to improve circuit configuration and layout.
Diplexers play an important role in dual band communication system, having a three-port circuit network for separating different frequency signals. Diplexers usually output high frequency signals and low frequency signals to different ports. In addition, diplexers also combine different frequency signals together. FIG. 1 schematically shows function blocks of a conventional diplexer. In FIG. 1, the diplexer comprises a low-pass filter 1 and a high-pass filter 3 and directs (or filters) input signal from antenna terminal A1, to port I/O(1) or I/O(2) according to frequency band of the input signal. When the input signal has higher frequency, the low-pass filter 1 is preferred to work almost as an open circuit, and therefore only the high-pass filter 3 dominates function of the diplexer to ensure no high frequency signal output to port I/O(1). Similarly, when the input signal has lower frequency, the high-pass filter 3 is preferred to work almost as an open circuit, and therefore only the low-pass filter 1 dominates function of the diplexer to ensure no low frequency signal output to port I/O(2).
FIG. 2 is a circuit diagram of a conventional diplexer. In FIG. 2, the diplexer comprises a low-pass filter 21 having an inductor L3, capacitors C6 and C8; and a high-pass filter 23 having inductors L4 and L5, and capacitors C7 and C9. The inductor L3 and capacitors C6 and C8 constitute a LC resonant circuit isolating signals of high frequency band and passing signals of low frequency band. The inductors L4 and L5 and capacitor C7 constitute a LC resonant circuit isolating signals of low frequency band and passing signals of high frequency band.
FIG. 3 is a circuit diagram of another conventional diplexer. In FIG. 3, the diplexer comprises a low-pass filter 31 having an inductor L6, capacitors C10 and C12; and a high-pass filter 33 having an inductor L7, and capacitors C11, C13 and C14. The inductor L6 and capacitor C10 constitute a LC resonant circuit isolating signals of high frequency band and passing signals of low frequency band. The inductor L7 and capacitor C11 constitute a LC resonant circuit isolating signals of low frequency band and passing signals of high frequency band.
FIG. 4 shows frequency response of the conventional diplexer depicted in FIG. 2 (or FIG. 3); wherein curves (a) and (b) respectively represent the frequency responses of high-pass filter and low-pass filter. In FIG. 4, first cut-off frequency (or resonant frequency) fp1 is determined in accordance with the inductor L3 and capacitor C6 in FIG. 2 (or the inductor L6 and capacitor C10 in FIG. 3). Second cut-off frequency fp2 is determined in accordance with the inductor L4 and capacitor C7 in FIG. 2 (or the inductor L7 and capacitor C11 in FIG. 3). The resonant frequency fp of LC resonant circuit is determined as
  fp  =            1              2        ⁢        π        ⁢                  LC                      .  Therefore, the lower the required resonant frequency (or cut-off frequency), the larger the required inductance L and capacitance C; this limits wire routing and circuit configuration on designing the diplexer.
Multilayer LTCC can be used to fabricate multilayer diplexer such that smaller inductance C and capacitance C are easily designed to obtain required resonant frequency in low frequency band and therefore reduces bulk of the diplexer.